The invention relates to an auto-bias developing process in an electrophotographic copying machine in which the surface potential of a photosensitive member having an electrostatic latent image formed thereon is detected by a detecting electrode through the interposition of a developer having a relatively low resistance and is utilized to determine, by an operational amplifier, a bias potential suitable to prevent a copy from scumming, which bias potential is applied to a developing electrode plate.
In the prior art, the detection of the potential of a latent image formed on a moving photosensitive member without contact therewith presented a considerable difficulty. For example, the weak electric field of the image must be accurately sensed and amplified with a high amplification factor and with an increased rate of response. For the most part, these problems have been successfully solved by the use of a developer having a relatively low resistivity (less than 10.sup.13 .OMEGA.-cm) to facilitate the detection of the potential of the latent image, and by improving the performance of the amplifier used. However, the auto-bias developing process developed thus far leaves much to be improved. For example, although a bias potential is applied to the developing electrode plate in accordance with the potential of the latent image formed, the originals to be reproduced such as the documents used for forming the latent image, vary widely, so that it is extremely difficult to produce a copy image free from scumming and of equal quality among the varying originals. Rather, it is recognized that copy images of a high quality and a uniform gradation can better be produced by employing a fixed bias potential rather than using the auto-bias which varies with the potential of the latent image.